[FEATURE]: Interactive /visualize command with multi-target diagramming

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Describe the enhancement you want to request

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Problem

When discussing complex distributed systems, execution lifecycles, or relational schemas, agents either dump 150+ lines of raw Mermaid/ASCII into the terminal or produce dense walls of text that are hard to visualize.

Proposal

Add an intent-aware /visualize command (opencode visualize) with an interactive confirmation step before rendering:

1. Confirmation Loop: Prompts the user with @clack/prompts to pick the target output (browser, terminal, ascii, file, or skip).
2. First-Class Targets: Supports context (System Context algebra & Session Runtime pipeline), architecture (monorepo layer topology), flow <module> (sequence diagrams), schema <path> (database ERD), deps (package graph), and state (Session drain lifecycle).
3. CLI HTML Artifact: Whenever run in CLI, generates an interactive dark-mode HTML canvas (.opencode/artifacts/visualize-*.html) with Mermaid.js, pan/zoom controls, and SVG download, presenting the clickable link to the user.